The proposed research is to correlate the histological and transmembrane potential characteristics of juxtanodal fibers (specialized fibers between the orifice of coronary sinus and the atrioventricular (AV) node). The technical approach is to record the transmembrane potentials of a juxtanodal fiber with microelectrode techniques and then study its electrophysiological properties. Then the fiber will be labelled by a special staining process. Then the tissue will be fixed, embedded, sectioned and stained. Histological characteristics of the fiber(s) from which action potentials are recorded shall be studied. Other physiological properties of juxtanodal fibers such as automaticity, its response to frequence of stimulation and functional syncytial relationship with AV node will be studied.